Various thermal transfer recording methods have hitherto been known in the art. Among others, a method for forming various full-color images using dyes for dye sublimation transfer as recording materials has been proposed. In this method, a thermal transfer sheet comprising dye layers formed by holding, by a suitable binder, dyes as recording materials on a base material such as a polyester film is provided, and the sublimable dyes are thermally transferred from the thermal transfer sheet onto a thermal transfer image-receiving sheet comprising a dye-receptive layer provided on an object dyeable with a sublimable dye, for example, paper or plastic film to form a full-color image. In this case, a large number of color dots of three or four colors with the quantity of heat being regulated are transferred by heating by means of a thermal head as heating means in a printer onto a receptive layer in the thermal transfer image-receiving sheet to reproduce a full color of an original by the multicolor dots. In this method, since coloring materials used are dyes, the formed images are very sharp and are highly transparent and thus are excellent in reproduction of intermediate colors and in gradation and are comparable with images formed by conventional offset printing or gravure printing. At the same time, this method can form high-quality images comparable with full-color images formed by photography.
In the thermal transfer recording method utilizing the thermal dye sublimation transfer, it has been pointed out that an increase in printing speed of thermal transfer printers has posed a problem that conventional thermal transfer sheets cannot provide satisfactory print density. Further, high density and high sharpness have become required of prints of images formed by thermal transfer. To meet this demand, various attempts have been made to improve thermal transfer sheets and thermal transfer image-receiving sheets which receive sublimable dyes transferred from the thermal transfer sheets to form images. For example, an attempt to improve the sensitivity in transfer at the time of printing has been made by reducing the thickness of the thermal transfer sheet. However, it has been pointed out that cockling occurs due to heat, pressure or the like applied at the time of the production of the thermal transfer sheet or at the time of thermal transfer recording and, in a few cases, breaking of the thermal transfer sheet occurs.
Further, an attempt to improve the print density and the sensitivity in transfer at the time of printing has been made by increasing the dye/binder ratio in the dye layer of the thermal transfer sheet. In this case, however, during storage in a wound state, the dye is transferred onto the heat-resistant slip layer provided on the backside of the thermal transfer sheet, and, at the time of rewinding, the dyes transferred onto the heat-resistant slip layer is retransferred onto dye layers of other colors or the like. That is, a kick back phenomenon occurs. When the contaminated dye layers are thermally transferred onto an image-receiving sheet, hue different from a designated one is provided, or otherwise the so-called “smudge” occurs. Further, in the image formation by thermal transfer, when high energy is applied in a thermal transfer printer, fusing of the dye layer to the receptive layer, that is, the so-called “abnormal transfer,” is likely to occur. A large amount of a release agent can be added to the receptive layer for abnormal transfer prevention purposes. However, it has been found that the addition of a large amount of the release agent causes blurring, smudge and other unfavorable phenomena of the image.
On the other hand, patent document 1 “Japanese Examined Patent Publication (Kokoku) No. 102746/1995” proposes a thermal transfer sheet wherein a hydrophilic barrier/subbing layer comprising polyvinylpyrrolidone as a main component and, mixed with the main component, polyvinyl alcohol as a component for enhancing dye transfer efficiency is provided between a dye layer and a support. When polyvinyl alcohol is used, the adhesion between the dye layer and the base material is unsatisfactory. Further, when polyvinylpyrrolidone and polyvinyl alcohol are used in such an addition amount as disclosed in Japanese Examined Patent Publication (Kokoku) No. 102746/1995, the adhesion is sometimes poor.
Japanese Patent Application No. 181812/2002 proposes a thermal transfer sheet that can realize an enhancement in sensitivity in the thermal transfer and can suppress abnormal transfer by using a polyvinylpyrrolidone-containing primer layer for a dye layer. As a result of a confirmative examination by the present inventors, however, it was found that, in this thermal transfer sheet, due to hygroscopicity by polyvinylpyrrolidone, particularly under high temperature and high humidity conditions, the adhesion of the primer layer is deteriorated and, at the time of thermal transfer, the dye layer is transferred in a layer form onto the image receptive layer in the image receiving sheet, or otherwise the separation and transfer, in a layer form, of the receptive layer onto the dye layer side, which are considered as derived from mixing of the primer layer with the dye layer, disadvantageously take place.
On the other hand, the present inventors have developed a thermal transfer sheet comprising an adhesive layer formed of a polyvinylpyrrolidone resin provided between the dye layer and the support and have filed a patent application (Japanese Patent Application No. 176982/2002). This thermal transfer sheet is advantageous in that the adhesive layer can enhance the efficiency of dye transfer onto the image receiving sheet to improve the print density and, at the time of printing, fusing to the image receiving sheet and abnormal transfer can also be suppressed. However, printing under a severe environment such as high humidity and use of an image receiving sheet having low releasability after long-term storage of this thermal transfer sheet comprising this adhesive layer under a high humidity environment such as an environment of 40° C. and 90% have sometimes caused fusing between the thermal transfer sheet and the image receiving sheet and abnormal transfer.
Accordingly, even when the thermal transfer printer and thermal transfer recording materials for the thermal transfer sheet and the thermal transfer image receiving sheet are regulated for meeting requirements for increased printing speed of the thermal transfer, increased density of thermally transferred images and higher quality, unfavorable phenomena take place including that no satisfactory print density can be provided and abnormal transfer occurs at the time of thermal transfer, making it impossible to provide printed matter having satisfactory quality.
Accordingly, at the present invention, the development of a thermal transfer sheet, which can meet requirements for increased printing speed of the thermal transfer, increased density of thermally transferred images and higher quality, and, at the same time, can produce printed matter having satisfactory quality, has been desired.